Ghostwalk is setting and rules book written by Monte Cook and I in 2001. It can be used as a self-contained campaign setting or as an add-on to an existing campaign. Although it was delayed, then canceled, WotC finally decided to print it (possibly because two d20 companies out there offered to buy it and that made WotC think it might sell a little bit) and it came out in 2003, just before the release of D& 3.5.

Though it suffers a bit for being neglected for so long and released right before D&D 3.5 (but now there's the 3.5 conversion document for the book, see above), and there are a couple of mistakes in it (a map was left out, probably because it was part of a poster map that was dropped from the book and it slipped through the cracks, but it's part of the 3.5 update file), I'm still really proud of it, as it's the first thing I was able to write for D&D that was allowed to be totally original and not based on an existing world. Monte and I were able to go a little crazy and have some fun with the setting and the game.

In short, the book's focus is a city called Manifest, which is built on top of the path to the land of the dead (the path is guarded by a clan of mysterious dwarves called the Deathwardens). Due to a quirk of the proximity of the land of the dead, ghosts can appear in the city and be solid and interact with living creatures. Unlike D&D ghosts, these ghosts are not undead and are basically just like other people, except they're made out of transparent ectoplasm. The premise of the setting is that if your character dies, you don't have to stop playing because you can just wait 10 minutes for your ghost to appear, grab your gear, sling your body over your shoulder, and keep going (or drag your body back to town to get a raise dead ... and I say raise dead because within the bounds of Manifest, getting raised from the dead doesn't cost you a level or a point of Constitution, even if you use a puny spell like raise dead. So people aren't really afraid of death. If you decide to adventure as a ghost, you can learn feats that let you possess people, make weird touch attacks, contort your ectoplasm into horrifying shapes, and other cool ghost powers.

The main bad guys in the setting are necromancers and undead, most of whom serve Orcus (yes, that Orcus), or yuan-ti, who hate ghosts because their souls aren't the kind kind to be able to form ghosts (when they die, they go right to the land of the dead). The yuan-ti are also trying to exterminate the elven race, and many elves come to Manifest to get advice from the Spirit Trees (which surround the city) which hold the ghosts of their ancestors.

Monte wrote the city section and so there are plenty of factions to join or antagonize, if you're into that sort of thing. The city is built on the ruins of two previous cities, so there are two entire buried cities below Manifest, which provide many spelunking opportunities for adventurers who want to hunt monsters.